This invention relates to imaging devices. More particularly, this invention relates to the placement of transitions in a data stream in an imaging device.
In an imaging device, such as an electrophotographic printer, copier, or fax machine, that uses a scanning device to expose a photoconductor, imaging data is used to control the application of current to a laser diode to form a latent electrostatic image on the surface of the photoconductor. The laser diode generates a beam that is swept across the surface of the photoconductor by the scanning device. The generation of high quality images can be accomplished by precisely controlling exposure of the photoconductor. The image is quantized into pixels. An improvement in image quality can be accomplished by decreasing the minimum quantization size of the area developed onto the photoconductor for a dimension of the developed area in the direction the beam is swept across the surface of the photoconductor. In addition to decreasing the minimum quantization size of the area developed, improved image quality is also accomplished by precisely controlling the positioning of the developed area with respect to the direction the beam is swept across the surface of the photoconductor. Decreasing the minimum quantization size can be accomplished by decreasing the minimum time period that the laser diode can be turned on during a sweep across the surface of the photoconductor.
In imaging devices using the beam to expose the photoconductor, the beam is generally passed through optical devices to condition it for exposing the photoconductor. After passing the beam through the optical devices, the beam is directed upon the surface of the photoconductor. Typically, the scanning device includes a rotating mirror surface (such as a rotating hexagonal scanning mirror) for sweeping the beam across the photoconductor. The rotation rate of the scanning mirror is tightly controlled. Therefore, the angular sweep rate of the beam will also be tightly controlled. However, because of the geometries involved, if the beam reflected from the scanning device is not conditioned, the sweep rate of the beam along the surface of the photoconductor will vary substantially across the surface of the photoconductor. The beam will sweep at a higher rate near the extremes of the sweep than it will at the central portion of the sweep. Without compensation for this effect, the central portion of the photoconductor can be exposed to a substantially higher optical energy density than the regions near the extremes of the sweep, thereby causing perceivable differences in the images formed on these regions.
To compensate for this sweep rate variation, a flat focusing lens is placed in the optical path between the scanning mirror and the photoconductor. The flat focusing lens is shaped to refract the beam so that the beam is substantially perpendicular to the surface of the photoconductor over length of the sweep and so that, over the length of the sweep, equal rotational movements of the scanning mirror will result in substantially equal movements of the beam across the surface of the photoconductor. By conditioning the beam in this manner, a substantial reduction in the variation of the optical energy density impinging upon the photoconductor over the length of the sweep can be achieved.
However, the flat focusing lens is, relative to the other optical devices in the photoconductor exposure system, costly to manufacture. As a result, the flat focusing lens accounts for a significant portion of the cost of the photoconductor exposure system. Therefore, there is a potential for a substantial cost savings in the imaging device if compensation for the sweep rate variations of the beam can be achieved without using the flat focusing lens.
Another difficulty that arises in imaging devices involves the memory required to store pixel data specifying the image o be formed on the media. Generally, data, such as print data, is used by the imaging device to generate pixel data specifying the image to be formed on the media. For example, an image rendered at a resolution of 1200 pixels per inch requires four times the pixel data to specify an image than the pixel data required to specify an image rendered at a resolution of 600 pixels per inch. Images can include regions having text and regions having pictures. Typically, the image to be generated from the print data is rendered at a uniform pixel density on the media and the rendered image is stored within the imaging device. However, it is generally true that the pixel density used to form relatively high quality text (with the characters having sharp edge definition) is higher than that used to form relatively high quality pictures. As a result, rendering the image at a uniform pixel density for both pictures and text can consume more memory space than would be required if the image could be rendered at a variable pixel density. Having the capability to vary the pixel density so that sections of the image including text are rendered at a higher pixel density than sections of the image including pictures reduces the memory required for storing the equivalent images at a uniform pixel density, thereby reducing cost.
Accordingly, in an imaging device, a method for generating a transition includes converting a first position of the transition relative to a synthesized clock to a second position of the transition relative to a clock. The method further includes determining an offset between the synthesized clock. Furthermore, the method includes shifting the second position based upon the offset and a phase difference between the clock and a reference signal to form a shifted position and generating the transition at the shifted position.
A variable resolution transition placement device includes a transform generator to generate a selectively variable transform value and a first converter to determine a first position of a transition relative to a clock using a second position of the transition relative to a synthesized clock and using the transform value. The variable resolution transition placement device also includes a synthesized clock generator configured to generate a synthesized clock value and a second converter configured to generate an offset value using the synthesized clock value. In addition, the variable resolution transition placement device includes an adder to combine the first position, the offset value, and a phase difference value to determine a cycle of the clock and a third position within the cycle to place the transition. Furthermore, the variable resolution transition placement device includes a transition queuing device to store and output the third position according to the cycle determined.
An imaging device includes a photoconductor, a rasterizer configured to generate pixel data, and a code generator to generate a binary encoded value corresponding to a position of a transition relative to a synthesized clock using the pixel data. In addition, the imaging device includes a variable resolution transition placement device. The variable resolution transition placement device includes a transform generator to generate a selectively variable transform value and a first converter to determine the position of the transition relative to a system clock using the binary encoded value and the transform value. In addition, the variable resolution transit on placement device includes a synthesized clock generator configured to generate a synthesized clock value, a second converter configured to generate an offset value using the synthesized clock value, and an adder to combine the position of the transition relative to the system clock, the offset value, and a pease difference value to determine a cycle of the system clock and an adjusted position within the cycle to place the transition. Furthermore, the variable resolution transition placement device includes a transition queuing device to store and output the adjusted position according to the cycle determined, transition generation logic to generate the transition according to the adjusted position, and a photoconductor exposure system configured to expose the photoconductor to light according to the transition.